KR102597626B1 - Transparent, essentially colorless, tin-fined LAS glass-ceramic with improved microstructure and thermal expansion properties - Google Patents

Abstract

The object of the present application is to develop a β-quartz glass-ceramic (with very interesting optical, thermal expansion and feasible properties); Products comprising the above glass-ceramics; It relates to precursor glasses of the glass-ceramics as well as to methods of elaborating the glass-ceramics and the products. The glass-ceramic has a composition free of arsenic oxides, antimony oxides and rare earth oxides, except for unavoidable traces, and containing the following components, expressed in percent by weight of oxides: 64 to 70% of SiO ₂ ; 18-24% Al ₂ O ₃ , 4-5% Li ₂ O, 0-0.6% SnO ₂ , >1.9-4% TiO ₂ , 1-2.5% ZrO ₂ , 0-1.5% MgO , 0 to 3% ZnO, >0.3 to 1% CaO, 0 to 3% BaO, 0 to 3% SrO, where BaO + SrO ≤ 3%, 0 to 1.5% Na ₂ O, 0 to 2% % K ₂ O, where 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤ 1, 0 to 3% P ₂ O ₅ , less than 250 ppm Fe ₂ O ₃ ; and their crystals, present in said β-quartz solid solution, have an average size of less than 40 nm, advantageously less than 35 nm, very advantageously less than 30 nm.

Description

Transparent, essentially colorless, tin-fined LAS glass-ceramic with improved microstructure and thermal expansion properties

The subject matter of this application is one of β-quartz glass-ceramics. This application, in particular:

- glass-ceramics of the lithium aluminosilicate (LAS) type, containing a solid solution of β-quartz as the main crystalline phase; The glass-ceramics contain in their composition neither As ₂ O ₃ nor Sb ₂ O ₃ and have very interesting properties: very interesting optical properties (transparency, absence of coloring and non-scattering properties), very interesting properties. Has thermal expansion properties and manufacturing characteristics (melting, forming) of great interest;

- Products made of the above glass-ceramic;

- Lithium aluminosilicate glasses, precursors of these glass-ceramics; As well as,

- Relates to a method of refining the glass-ceramic and products made of the glass-ceramic.

Low thermal expansion glass-ceramics containing a solid solution of β-quartz as the main crystalline phase (often called β-quartz glass-ceramics) are used in, for example, cooktops, cookware, and microwave oven soles. Useful as fireproof glazing, chimney windows, fireplace inserts, stove and oven windows, especially those of pyrolytic or catalytic ovens, shieldings, fireproof glazings, especially incorporated into doors or windows or used as partitions. . These glass-ceramics can be colored (e.g. black cooktops) or transparent and colorless (e.g. refractory glazing, cooking hobs for induction heat (preferably with a fully visible colored lower layer)). hobs), stove and oven windows and shields).

To obtain these glass-ceramics (more specifically to remove gaseous inclusions in the bulk of the precursor molten glass), conventional fining agents: As ₂ O ₃ and/or Sb ₂ O ₃ has been used in conventional glass-ceramic compositions. The use of these conventional fining agents is mainly exemplified in patent documents US 4,438,210, US 5,070,045 and WO 2005/058766. In fact, given the toxicity of As ₂ O ₃ and increasingly stringent regulations, the use of this toxic fining compound is no longer required to make precursor glasses. For environmental considerations, the use of halogens such as F and Br, which can be used to replace, at least partially, the conventional fining agent As ₂ O ₃ and the use of Sb ₂ O ₃ are also no longer required.

SnO ₂ can be used as an alternative fining agent (to As ₂ O ₃ and Sb ₂ O ₃ ). Patent application US 2011/0071011 and patents US 6,846,760 and US 8,053,381 describe glass-ceramic compositions containing SnO ₂ as fining agent.

However, the use of SnO ₂ as a fining agent has disadvantages. For example, this compound is more inefficient than As ₂ O ₃ (and in absolute terms, therefore, it must be used in relatively large quantities to offset the inefficiency, and the problems it poses, in particular the problem of devitrification, are avoided). no), and this is the cause of the occurrence of undesirable yellowish coloration during ceramming. This yellowish coloration is particularly undesirable if the goal is to obtain a transparent and essentially colorless glass-ceramic (the present application relates to obtaining a transparent and essentially colorless glass-ceramic, but patent application EP 2 088 130 and WO 2010/136731 relate to colored glass-ceramics). This yellowish coloration results from Ti-Fe interactions (due to charge transfer), and these interactions were observed to increase in the presence of tin. Precursor glass compositions of glass-ceramics generally contain TiO ₂ , as a nucleation agent, and also iron, provided as an impurity (e.g. by raw materials and glass cullet). Accordingly, a need exists for a transparent and essentially colorless glass-ceramic composition that avoids this undesirable yellowish coloration phenomenon.

US Pat. No. 8,053,381 and US Pat. No. 8,685,873 (already mentioned above) describe the use of expensive color compensation agent(s) that are detrimental to the light transmission of glass-ceramics.

US Patent No. 8,759,239 and US Patent No. 8,143,179 describe limiting or avoiding the presence of TiO ₂ in the composition of the precursor glass.

Application WO 2013/171288 describes a transparent, essentially colorless and non-scattering β-quartz glass-ceramic. Their compositions contain SnO ₂ as fining agent and no MgO or only a low content of MgO.

Patent applications WO 2012/020678 and WO 2012/066948 describe glass-ceramics, the precursor glass of which is fined with SnO ₂ , used for example as oven windows.

In this situation, the applicant proposes a new β-quartz glass-ceramic, the composition of which therefore contains neither As ₂ O ₃ nor Sb ₂ O ₃ (the composition does not contain any halogens). These new glass-ceramics have optical properties, in particular the properties of light transmission, non-scattering of light and low residual staining, which are of great interest. This new glass-ceramic is transparent, essentially colorless and non-scattering. These new glass-ceramics are easy to obtain, even on an industrial scale, because their precursor glasses have low viscosity, low liquidus temperature and high liquidus viscosity at high temperatures, which are compatible with their formation methods. , and can be crystallized by carrying out a ceramization cycle of short duration (the presence of an effective amount of nucleating agent, especially TiO ₂ in their composition is appropriate). With regard to the optical properties and the easy conditions for obtaining them, the excellent results obtained are on the level of those obtained with the glass-ceramics described in application WO 2013/171288. However, obtaining these excellent results is based on a completely different approach (see later), and this is noteworthy, especially in that additional conditions (related to CTE) have to be taken into account. In addition to their very interesting optical properties and the ease of obtaining them, the glass-ceramics of the present application also have very interesting thermal expansion properties. Accordingly, the new glass-ceramic of the present application complies with a number of conditions, in particular a specification comprising the following conditions:

- Obtaining them from inexpensive raw materials (their composition is free from any expensive exotic elements and, in particular, rare earth oxides, such as Nd ₂ O ₃ ),

- compositions free of any As, Sb, halogens,

- Optical properties of very interest: high transparency (luminous transmittance (TL) of >81%, or even >84%, for a thickness of 5 mm), very low yellow index (YI) (= essentially colorless) (for a thickness of 5 mm, less than 14, or even less than 12), and low scattering level (haze percentage of less than 2.5%, or even less than 1.5% for a thickness of 5 mm),

- Ease of obtaining them: low viscosity at high temperatures of their precursor glasses (T _@30Pa.s <1,640°C), low liquidus temperature (<1,400°C), high liquidus (of the precursor glasses) linear viscosity (typically >300 Pa.s), and a possible ceramization cycle (of the precursor glass) of short duration (less than 3 h), and

- Low CTE _{25℃ -[300-700℃]} (between ± 3.5.10 ⁷ K ^-1 ). It should be noted that these conditions regarding coefficient of thermal expansion (CTE) are more stringent than usual. This is imposed at any temperature between 25°C and 300 to 700°C. For this matter, the attached Figure 1 may be considered. This condition is particularly demanding in relation to certain applications (of glass-ceramics) that require resistance to thermal shock in different temperature ranges.

According to the first object, the present application relates to the following glass-ceramics:

- glass-ceramics of the lithium aluminosilicate type (LAS): these contain Li ₂ O, Al ₂ O ₃ and SiO ₂ as essential components of the β-quartz solid solution (see above);

- Glass-ceramics containing β-quartz solid solution as the main crystalline phase: β-quartz solid solution represents more than 80% by weight of the total crystallized fraction. This usually represents even more than 90% by weight of the total crystallized portion; and

- Glass-ceramics that comply in a particularly satisfactory manner with the above specifications (in particular the conditions of said specifications relating to optical properties and CTE).

In a characteristic way:

- The composition of the glass-ceramics of the present application, which is free from arsenic oxides, antimony oxides and rare earth oxides, except for unavoidable traces, contains the following components, expressed in percent by weight of oxides:

64 to 70% SiO ₂ ,

18 to 24% Al ₂ O ₃ ,

4 to 5% Li ₂ O,

0 to 0.6% SnO ₂ ,

>1.9 to 4% TiO ₂ ,

1 to 2.5% ZrO ₂ ,

0 to 1.5% MgO,

0 to 3% ZnO,

>0.3 to 1% CaO,

0 to 3% BaO,

0 to 3% SrO, where BaO + SrO ≤ 3%,

0 to 1.5% Na ₂ O,

0 to 2% K ₂ O, where 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤ 1,

0 to 3% of P ₂ O ₅ ,

Fe ₂ O ₃ less than 250 ppm, and

- The crystallites present in the β-quartz solid solution (the majority crystalline phase) have an average size of less than 40 nm, advantageously less than 35 nm, very advantageously less than 30 nm.

Figure 1 illustrates the change in coefficient of thermal expansion (CTE) between 25 (°C) and temperature T (°C) with respect to temperature (T) for the glass-ceramics of the prior art and the glass-ceramics of the present application.

This concept of average crystallite size is familiar to those skilled in the art. Traditionally, this is measured using Rietveld refinement of the X-ray diffraction spectrum.

The composition of the glass-ceramic (of type LAS) of the present application contains:

- 64 to 70% SiO ₂ ,

- 18 to 24% Al ₂ O ₃ , and

- 4 to 5% Li ₂ O.

The SiO ₂ content (≥64%) should be suitable to obtain a sufficiently viscous precursor glass to limit devitrification problems. The SiO ₂ content is limited to 70% because the higher the SiO ₂ content, the more difficult it is to melt the composition. The content is advantageously between 64 and 68% (inclusive limits).

With regard to Al ₂ O ₃ , excess amounts (>24%) are undesirable as they may lead to further devitrification of the composition (mullite or other crystalline phases). Conversely, too small an amount (<18%) is detrimental to nucleation and formation of small β-quartz crystals. A content of 20 to 24% (inclusive limit) is advantageous.

Li ₂ O: Excessive amounts (>5%) are beneficial for devitrification, while too small amounts (<4%) significantly increase the high temperature viscosity. Contents above 4% are advantageous. In a characteristic way, the glass-ceramics of the invention therefore contain a significant amount of Li ₂ O (4-5%, advantageously >4-5%), especially in connection with their low viscosity at high temperatures.

The glass-ceramic composition of the present application contains neither As ₂ O ₃ nor Sb ₂ O ₃ or contains only a trace amount of at least one of these toxic compounds; SnO ₂ is generally present instead of and as a substitute for these conventional fining agents (see below). If at least one of these compounds is present in trace amounts, it is present as an impurity. This is due to the presence in the vitrifiable charge (=batch mixture) of the raw materials, for example of recycled materials (e.g. old glass-ceramics refined with these compounds). In any case, only trace amounts of toxic compounds may be present: As ₂ O ₃ + Sb ₂ O ₃ <1,000 ppm. Similar comments apply to halogens. Halogens are not used as raw materials. Only trace amounts of halogens may be present (less than 1,000 ppm).

The composition of the glass-ceramic of the present application contains colorants (oxides), such as certain rare earth oxides, namely Nd ₂ O ₃ , which, in the presence of SnO ₂ as fining agent, can ensure the role of decolorizing or compensating colorants. I never do that. The unavoidable trace amounts that may be present do not correspond to the effective amounts with respect to decolorization.

To obtain the sought results - especially the very interesting light transmission, non-scattering and low residual coloring properties - in the presence of the (chemical) fining agent, said SnO ₂ , the inventors propose an original approach:

- differs from those according to US Pat. No. 8,053,381 and US Pat. No. 8,685,873 (see above) in the absence of rare earth oxides,

- the composition of the glass-ceramic of the present application differs from that according to patents US 8,759,239 and US 8,143,179 in that it contains a significant amount of TiO ₂ , and also

- Different from those according to applications WO 2013/171288, WO 2000/020678 and WO 2012/066948.

The composition of the glass-ceramic of the present application contains:

- In general, an effective amount of fining agent: 0.1% to 0.6% SnO ₂ (incidentally, the present application covers glass-ceramics that do not contain SnO ₂ or have small amounts (<0.1%) of SnO ₂ in their composition. It is noteworthy here that it is inclusive). The precursor glasses of the glass-ceramics can be significantly refined thermally or essentially thermally. However, those skilled in the art will recognize that thermal fining (or essentially thermal fining), especially on an industrial scale, is much more demanding to apply than thermal and (with SnO ₂ ) chemical fining, and therefore the glass-ceramics of the present application. It is recognized that this generally includes an effective amount of SnO ₂ as a fining agent, together with the problems that the presence of SnO ₂ implies (see above), and

- Effective amounts of nucleating agents: >1.9% to 4% TiO ₂ and 1 to 2.5% ZrO ₂ (see below).

The inventors' ingenious approach (particularly in relation to the numerous conditions of the specifications listed above) is: 1) With the conditions listed for the (MgO + Na ₂ O + K ₂ O)/Li ₂ O ratio, a substantial amount of Li It is essentially based on the combined presence of ₂ O and CaO, 2) on the presence of low Fe ₂ O ₃ content (less than 250 ppm), and 3) on the presence of small β-quartz crystals,

1) The present inventors have found the advantage of combining Li ₂ O, CaO, MgO, Na ₂ O and K ₂ O in the indicated amounts.

2) Fe ₂ O ₃ content of <250 ppm offers the possibility to limit Ti-Fe interactions, thereby minimizing color and maximizing light transmission.

3) It has also been observed that the small average size of the β-quartz crystals (<40 nm, advantageously <35 nm, very advantageously <30 nm) is advantageous for obtaining high light transmission, low coloration and low scattering levels. The small average size of these crystals is due to the quality of nucleation and, therefore, to the presence of nucleating agents: TiO ₂ and 1 to 2.5% by weight ( Advantageously in an amount of 1.5 to 2% by weight), ZrO ₂ , and the ceramization heat treatment are associated. In addition, the fining agent, SnO ₂ (commonly present, see above), is also known to be involved in the nucleation process.

As for the low scattering level (haze percentage) exhibited by the glass-ceramics of the present application, this is also related not only to the size of the crystals, but also to their number. This also depends on the quality of the nucleation and therefore on the presence of the nucleating agent and on the ceramicizing heat treatment.

Accordingly, in a characteristic manner the glass-ceramics of the present application exhibit:

- an average size of less than 40 nm, advantageously less than 35 nm, very advantageously less than 30 nm (these values can be compared with the values 40-61 nm given for representative crystals of glass-ceramics in US Pat. No. 6,846,760 ) (predominantly in the crystalline phase, present in β-quartz solid solution), and

- A composition free _of As ₂ O ₃ , Sb ₂ O 3 , halogens and rare earth oxides containing (in addition to SiO ₂ , Al ₂ O ₃ and Li ₂ O in the weight percentages indicated above):

+ Advantageously, in an amount of 0.1 to 0.6% by weight (very advantageously 0.1 to 0.4% by weight), as fining agent, SnO ₂ : The presence of SnO ₂ (in the absence of conventional fining agents) helps in the application of fining. Favorable in relation to (see above). As a (chemical) fining agent, SnO ₂ also contributes to nucleation. However, this intervenes in limited amounts (≤0.6%) in connection with the technical problem of glass-ceramic (“yellowing”) coloration (see above) and also in connection with the technical problem of devitrification at high temperatures. The inventors have noted here that the present application also covers glass-ceramics that are free of SnO ₂ or have a small amount (<0.1%) of SnO ₂ in their composition. The glass-ceramic is refined thermally or essentially thermally. Their highly satisfactory optical properties are obtained without any difficulty. Very interestingly, they meet other conditions of the above specification;

+ as nucleating agent, in an amount of more than 1.9 to 4% by weight (advantageously 2 to 3% by weight) relative to TiO ₂ and in an amount of 1 to 2.5% by weight (advantageously 1.5 to 2% by weight) relative to ZrO ₂ , TiO ₂ and ZrO ₂ . The presence of ZrO ₂ makes it possible to limit the presence of TiO ₂ . The TiO ₂ is present in an adequate amount for the desired effect on nucleation (within a reasonable period of time), but in a limited amount in connection with the technical coloration issues (described above). The ZrO ₂ completes the action of the TiO ₂ for nucleation, but cannot intervene in large quantities as it then creates devitrification problems. In the composition of the glass-ceramic of the present application, in relation to the size of the crystals and the period of ceramization, the total amount of nucleating agent (TiO ₂ + ZrO ₂ ) is suitably greater than 3.8% by weight (>3.8%), advantageously at least 4% by weight (≥4%), advantageously at least 4.2% by weight (≥4.2%), in fact at least 4.5% by weight (≥4.5%);

+ MgO: is present in an amount of 0 to 1.5% by weight (advantageously at least 0.1% by weight, very advantageously 0.1 to 0.5% by weight). MgO advantageously intervenes together with CaO and also Na ₂ O and K ₂ O to obtain the desired values of thermal expansion coefficient of interest (see listed conditions). If present in excess of 1.5% by weight, it causes the value of the coefficient of thermal expansion to be too high and yellowish coloration;

+ ZnO: is present in an amount of 0 to 3% by weight (advantageously at least 0.1% by weight, very advantageously 0.1 to 1.5% by weight). ZnO advantageously reinforces the action of Li ₂ O to achieve low viscosity at high temperatures. ZnO should not be used in more than 3% by weight, with respect to the optical properties and the desired coefficient of thermal expansion value.

It can be seen from above that MgO and ZnO are each independently involved, advantageously at least 0.1% by weight. In any case, it is highly recommended that the minimum MgO + ZnO represents at least 0.1% by weight;

+ CaO: This component is present in significant amounts (more than 0.3% by weight), especially with regard to the desired value of the coefficient of thermal expansion, and does not exceed (maximum 1% by weight), especially with regard to the sought-after optical properties. No. It also intervenes with Na ₂ O to obtain a low liquidus temperature of the precursor glass and with Na ₂ O and K ₂ O to achieve low viscosity at high temperatures of the precursor glass. It is advantageously present at 0.4 to 0.7% by weight;

+ BaO and SrO: each present in an amount of 0 to 3% by weight (advantageously 0.5 to 1.5% by weight each). Since SrO is generally an expensive product, it does not exist as an added raw material. In these situations (where SrO is not present as an added raw material), if SrO is present, it is added only in unavoidable trace amounts (<1000 ppm) as an impurity contained in at least one of the raw materials used or in the cullet used) . Excess BaO and/or SrO (more than 3% by weight) can have a high residual glass content, resulting in a glass-ceramic with a yellowish coloration;

+ Na ₂ O and K ₂ O: present in amounts of 0 to 1.5% by weight for Na ₂ O and 0 to 2% by weight for K ₂ O, respectively. If these indicated maximum amounts are exceeded, the coefficient of thermal expansion becomes too high, and the desired optical properties are impaired (incidence of color and scattering). The presence of one or both of these elements is not required, but it does ensure that the condition 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤ 1 is consistent with the conditions in the specification associated with the coefficient of thermal expansion above. In this regard, it was mentioned that observations should be made. With regard to the low liquidus temperature of the glass-ceramics of the present application, it may be noted that Na ₂ O is also advantageously involved with CaO;

+ P ₂ O ₅ : Present in an amount of 0 to 3% by weight. P ₂ O ₅ usefully intervenes as a manufacturing aid to promote dissolution of ZrO ₂ and limit devitrification. However, this may impair the optical properties and its presence is generally lower than 1% by weight and in any case is limited to 3% by weight. According to an advantageous alternative, the composition of the glass-ceramic according to the present application contains (with the explicit exception of unavoidable trace amounts (<1000 ppm) which may enter as impurities from at least one of the raw materials used or from the cullet used) P ₂ O ₅ There is no;

+ Fe ₂ O ₃ : less than 250 ppm (advantageously less than 200 ppm, very advantageously less than 180 ppm; this is generally difficult to be less than 100 ppm due to the presence of iron in the raw materials used). In fact, the compositions of the glass-ceramics of the present application generally contain from 100 to less than 200 ppm Fe ₂ O ₃ . It is understood here that the Ti-Fe transfer of charges (which is to be minimized or even avoided), causing coloration, should be limited. It is emphasized that the glass-ceramics of the present application have very interesting optical properties without requiring very low Fe ₂ O ₃ levels in their compositions.

With regard to the specific conditions for the coefficient of thermal expansion (low CTE _{25°C -[300-700°C]} (between ± 3.5. 10 ⁷ K ^-1 ), this is due to the presence of CaO and Li ₂ O, MgO, Na ₂ O and K It is understood that ₂ O satisfies the condition 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤ 1. For this ratio, below the value of 0.2, the glass-ceramic has a too negative coefficient of thermal expansion ( < -3.5.10 ⁷ K ^{- 1} ); if a value of 1 is exceeded for this ratio, the glass-ceramics have too high a coefficient of thermal expansion (> 3.5.10 ⁷ K ^{- 1} ).

According to a particularly advantageous alternative, the composition of the glass-ceramic of the present application, free from arsenic oxide, antimony oxide, rare earth oxides, strontium oxide and phosphorus oxide, except for unavoidable traces, comprises the following components, expressed in percent by weight of the oxides: Contains:

64 to 68% SiO ₂ ,

20 to 24% Al ₂ O ₃ ,

>4 to 5% Li ₂ O,

0.1 to 0.4% SnO ₂ ,

2 to 3% TiO ₂ ,

1.5 to 2% ZrO ₂ ,

0.1 to 0.5% MgO,

0.1 to 1.5% ZnO,

>0.3 to 1% CaO,

0.5 to 1.5% BaO,

0 to 1.5% Na ₂ O,

0 to 2% K ₂ O, where 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤ 1, and

Fe ₂ O ₃ less than 200 ppm.

Compositions of the glass-ceramics of the present application, as defined above (SiO ₂ , Al ₂ O ₃ , Li ₂ O, SnO ₂ , TiO ₂ , ZrO ₂ , MgO, ZnO, CaO, BaO, SrO, Na ₂ O, K ₂ O, P ₂ O ₅ and Fe ₂ O ₃ ), which are or may be introduced, represent substantially 100% by weight of the composition of the glass-ceramic of the present application, but also (their optical properties of interest, At least one other compound may be present in small amounts (up to 3% by weight) that cannot be completely excluded a priori, without substantially affecting the properties of the glass-ceramic (or their low CTE). The following compounds: Nb ₂ O ₅ , Ta ₂ O ₅ , WO ₃ and MoO ₃ may in particular be present in a total content of up to 3% by weight, each of which has a total content of up to 2% by weight. In connection with the absence of coloring sought (a very low yellowing index is sought), colorants, such as V ₂ O ₅ , are obviously not used as raw materials for the glass-ceramics of the present application. Regarding the prohibited use of the above colorants, however, an exception (and only one exception) is made for CoO. CoO may be present (added) to optimize optical properties. CoO is an inexpensive colored oxide (it is not a rare earth oxide) and its presence in very small amounts (≤ 30 ppm, typically ≤ 10 ppm) can further improve the already very small yellowing index. The presence of CoO in excess of 30 ppm gives the glass-ceramic a pink color and reduces light transmission.

Compositions of the glass-ceramics of the present application, as defined above (SiO ₂ , Al ₂ O ₃ , Li ₂ O, SnO ₂ , TiO ₂ , ZrO ₂ , MgO, ZnO, CaO, BaO, SrO, Na ₂ O, K ₂ O, P ₂ O ₅ and Fe ₂ O ₃ ), thus, at least 97% by weight, or at least 98% by weight, or even at least 99% by weight, or even 100% by weight of the composition of the glass-ceramic of the present application. Indicates % (see above).

It has actually been mentioned that the glass-ceramics of the present application are of the lithium aluminosilicate (LAS) type and that they contain β-quartz solid solution as the main crystalline phase; The β-quartz solid solution represents more than 80% by weight of the total crystallized portion. In fact, the β-quartz solid solution generally represents more than 90% by weight of the total crystallized portion. The composition of the glass-ceramic is free of As ₂ O ₃ , Sb ₂ O ₃ and rare earth oxides (and also halogens), except for unavoidable traces.

Accordingly, the glass-ceramics of the present application satisfactorily meet the other conditions of the specification, listed above. The transparent, essentially colorless and non-scattering glass-ceramics of the present application (precursor glasses advantageously purified with an effective amount of SnO ₂ ) thus have the optical properties mentioned below:

- for a thickness of 5 mm, a total luminous transmittance of greater than 81%, advantageously greater than 84%; This parameter (expressed as TL,%) quantifies transparency. This is known to those skilled in the art. This is defined by the ASTM D1003-13 standard. The standard total luminous transmittance measurement covers the spectral range of 380-780 nm;

- yellowing index of less than 14, advantageously less than 12 (and very advantageously less than 10), for a thickness of 5 mm. This parameter (YI), known to those skilled in the art, quantifies the intensity of residual yellowing staining. Values of this index below 14 characterize glass-ceramics with very low residual coloring. The equation for calculating this index, known to those skilled in the art, is: YI _{ASTM D1925} = [100 x (1.28X-1.06Z)]/Y, where ) represents the tristimulus coordinates of the sample, calculated relative to the observer at C and 2°, and

- a haze percentage (which measures the level of scattering) of less than 2.5%, advantageously less than 1.5%, for a thickness of 5 mm. This is understood as the lower the scattering, the better the appearance (and therefore optical quality) of the material. Haze is calculated in the following manner: Haze (%) = (T _Diffusion /T _Total )x100, where T _Diffusion is the diffuse transmission (%) and T _Total is the total transmission (%). Haze measurements are performed according to the ASTM D1003-13 standard (using an integrating sphere). This is known to those skilled in the art. A person skilled in the art, or even a lambda witness, knows how to recognize the diffusive or non-diffusive nature of a substance, in any case, with the naked eye.

The glass-ceramic of the present application has a low coefficient of thermal expansion (CTE), more specifically ± 3.5. It may further have a coefficient of thermal expansion (CTE) between 25°C and any temperature between 300 and 700°C (CTE _{25°C -[300-700°C]} ) at 10 ⁷ K ^-1 .

In this regard, the glass-ceramics of the present application are of particular interest. Moreover, their ease of obtaining has been mentioned taking into account the properties of the precursor glass (see above).

According to a second object, the present application relates to a product made at least partly, advantageously entirely, of the glass-ceramic of the invention as described above. The product advantageously consists entirely of the glass-ceramic of the present application. The products are, inter alia, chimney windows, fireplace inserts, stove and oven windows, especially windows of pyrolytic or catalytic ovens, cooktops (preferably for heating by induction with a fully visible, colored lower layer), It may be in a shield, or in fire-resistant glazing (especially fire-resistant glazing incorporated into doors or windows or used as partitions). Of course, the glass-ceramics of the present application are fully understood to be used a priori logically in the context of their appropriate optical properties and their advantageous thermal expansion properties.

According to a third object, the present application relates to lithium aluminosilicate (LAS) glass, which is a precursor of the glass-ceramics of the present application, as described above. The glass has a composition that, in a unique way, provides the possibility of obtaining the glass-ceramic. The glass generally has a composition corresponding to that of the glass-ceramic, but this matching means that the heat treatment imposed on the glass to obtain the glass-ceramic may have some influence on the composition of the material. As long as it is fully recognized by those skilled in the art, it is not necessarily complete.

According to the advantageous variant:

- the composition of said glass contains at least 0.1% ZnO, advantageously between 0.1 and 1.5% ZnO; and/or

- the composition of the glass contains less than 1% of P ₂ O ₅ and the composition of the glass is advantageously free of P ₂ O ₅ , except for unavoidable traces.

These glasses are of particular interest in that they have advantageous high-temperature viscosity (low viscosity) as well as interesting devitrification properties, which are compatible with the application of forming methods by rolling and floating. It can be seen from the above that the glass has a low high temperature viscosity (T _@30Pa.s <1,640°C), a low liquidus temperature (<1,400°C), and a high liquidus viscosity (>300 Pa.s). Moreover, from the precursor glasses of the present application, it is possible to obtain the glass-ceramics of the present application by applying ceramization cycles of short duration (less than 3 hours). The glasses of the present application are traditionally obtained by melting (in appropriate proportions) the raw materials introduced into their composition.

According to the fourth and fifth objects, the present application provides, respectively, a method for elaborating a glass-ceramic of the present application, as described above, and an article consisting at least partially of the glass-ceramic of the present application, as described above. It's about method.

The above methods are similar methods.

Traditionally, the methods for refining said glass-ceramics include melting, fining (thermal fining (SnO ₂ = 0) (or essentially thermal fining (SnO ₂ <0.1%)), advantageously thermal and chemical fining (SnO ₂ ≥ 0.1%)) and a batch mixture capable of vitrification of the raw materials, advantageously containing SnO ₂ as fining agent, under conditions of ensuring successively the ceramization by the first nucleation step and the second crystal growth step. Including heat treatment. Both the successive steps for obtaining the fined glass (precursor of glass-ceramics) and for ceramicizing the fined glass can be applied sequentially with each other or can be moved in time (to the same place or to another place).

In a characteristic way, the vitrifiable (= vitrifiable) batch mixture of the raw materials has a composition that provides the possibility of obtaining the glass-ceramic of the present application, having the weight composition indicated above, the ceramization being carried out as follows: Applies as follows:

- during the nucleation step, at a temperature interval of 650 to 850° C. for 15 minutes to 4 hours, and

- During the crystal growth phase, at a temperature interval of 860 to 950° C., for 10 minutes to 2 hours.

Ceramization applied under the above conditions for glasses with the indicated composition leads to the expected results, especially in terms of the size of the β-quartz crystals.

Within the scope of the present application, optimization of the optical properties of glass-ceramics can be obtained by acting on the parameters of the correct composition charging and ceramization cycle.

Traditionally, the way a product is elaborated is:

- melting a batch mixture capable of vitrification of the raw materials, said batch mixture advantageously containing SnO ₂ as fining agent; followed by fining the obtained molten glass (thermal fining (SnO ₂ = 0) (or essentially thermal cleaning (SnO ₂ <0.1%), advantageously thermal and chemical fining (SnO ₂ ≥0.1%));

- Cooling the obtained clarified molten glass and at the same time shaping it into the desired shape for the target product;

- During ceramization of the shaped glass, a heat treatment step comprising a first nucleation step and a second crystal growth step is sequentially included.

All of these sequential steps for obtaining fined shaped glass (precursor of products in glass-ceramics) and for ceramicizing said fined shaped glass can be applied sequentially with each other or in time (at the same place or at different places). can be moved to

In a characteristic way, the vitrifiable (= vitrifiable) charge of the raw material has a composition that provides the possibility of obtaining the glass-ceramic of the invention, having a weight composition as indicated above, the ceramizing heat treatment :

- during the nucleation step, at a temperature interval of 650 to 850° C., for 15 minutes to 4 hours, and

- During the growth crystal phase, it is applied for 10 minutes to 2 hours, at temperature intervals of 860 to 950° C.

Ceramization applied under the above conditions for glasses with the indicated composition leads to the expected results, especially in terms of the size of the β-quartz crystals.

Within the scope of the present application, optimization of the optical properties of glass-ceramics can be obtained by acting on the parameters of the filling and ceramization cycles of the composition.

Incidentally, it has been mentioned herein that this method may or may not use an effective amount of SnO ₂ . Effective amounts of SnO ₂ (0.1 to 0.6%: see above) are advantageously used.

General comments may be stated below, with regard to the ceramic cycle indicated.

The ceramizing heat treatment, with the above characteristics, consists of nucleation (nucleation step applied at at least 650°C) and obtaining a glass-ceramic containing β-quartz solid solution as the main crystalline phase (crystal growth step applied at a temperature up to 950°C). ) is guaranteed.

If the nucleation temperature interval is inadequate (i.e. outside the 650-850°C range) or if the time in this interval is too short (less than 15 minutes), an insufficient number of seeds will be formed and the subsequent material tends to spread.

Moreover, if the growth temperature is too low (i.e., <860°C), the resulting glass-ceramics tend to have large scattering, and conversely, if the growth temperature is too high (i.e., >950°C), the resulting glass-ceramics are opaque. It tends to break down.

It should be noted here that obtaining the glass-ceramics of the present application from precursor glasses shaped by floating is not excluded. However, the float glass method (floating) is not preferred because it is usually performed for high production volumes and can be detrimental to the light transmission of the produced glass-ceramic. The inventor recommends other shaping methods such as rolling.

Hereinafter, this application will be illustrated by the following examples.

The attached figure 1 shows a glass-ceramic of the prior art (hereinafter referred to as glass-ceramic of Comparative Example A (= glass-ceramic according to WO 2013/171288)) and a glass-ceramic of the present application (hereinafter referred to as glass-ceramic of Example 2). For ceramics), versus temperature (T), the change in coefficient of thermal expansion (CTE) between 25 (°C) and temperature T (°C) is illustrated. Considering the two curves shows the advantages of the present application:

- The glass-ceramic of Example A has a CTE of 25 to 700° C., which is the range sought (CTE _{25° C.-700° C.} = -2.1x10 ^-7 K ^{- 1} ), but a CTE of 25 to 300° C., which is too low ( CTE _{25℃ -300℃} = -6.2x10 ^-7 K ^-1 ) has:

- The glass-ceramic of Example 2 has an appropriate CTE (CTE _{25°C -700°C} = 1.2x10 ^-7 K ^-1 and CTE _25°C-300°C = -2.6x10 ^-7 K ^{-1 )} .

According to Tables 1 and 2 below, the glass-ceramics of the present application have suitable CTE _{25°C -700°C} and CTE _{25°C -300°C} values. This is due to the strict control of their composition and the combined effect of the amounts of their different constituent elements (Li ₂ O, CaO, MgO, Na ₂ O and K ₂ O are especially relevant).

Example

· To produce a batch of 1 kg precursor glass, the raw materials are carefully mixed, in the proportions (proportions expressed as oxides) reported in Tables 1 and 2 below, and in the first part of Tables 3 and 4.

The mixture is placed for melting in a platinum crucible. The crucible containing these mixtures is then introduced into an oven preheated to 1,550°C. There, they apply to the following types of melting cycles:

- Raising the temperature from 1,550°C to 1,670°C within 1 hour;

- Maintain them at 1,670℃ for 5 hours and 30 minutes.

The crucible is then taken out of the oven and the molten glass is poured onto a preheated steel plate. This is layered on top of it up to a thickness of 6 mm. A glass plate is thereby obtained. They are annealed at 650°C for 1 hour and then slowly cooled. · The properties of the obtained glass are shown in the second part of Tables 1 and 2, and Tables 3 and 4 below.

Viscosity is measured with a rotational viscometer (Gero).

T _30Pa.s (°C) corresponds to the temperature _at which the viscosity of glass is 30 Pa.s.

T _liq (℃) is the liquidus temperature. In fact, the liquidus line is given by a range of associated temperatures and viscosities: the highest temperature corresponds to the lowest temperature at which no crystals are observed, and the lowest temperature corresponds to the highest temperature at which crystals are observed.

The devitrification characteristics (low and high liquidus temperatures) are determined in the following manner. Glass samples (0.5 cm3) are subjected to the following heat treatment:

- Introduced into an oven preheated to 1,430℃,

- Maintain this temperature for 30 minutes,

- cooling to test temperature T, at a rate of 10°C/min,

- Maintain at this temperature for 17 hours,

- Quenching of samples.

Any possible crystals present are observed under an optical microscope.

· The applied ceramicizing cycle is specified below:

- Raising temperature from room temperature (25°C) to 650°C at a heating rate of 30°C/min;

- Raise the temperature from 650° C. to 820° C. in 40 minutes (ramp of 4.3° C./min);

- Raising the temperature from 820° C. to 900° C. in 17 minutes (ramp of 4.7° C./min);

- Maintaining the temperature of 900°C for 15 minutes,

- Lowering the temperature to the thermal inertia of the oven.

· The properties of the obtained glass-ceramic are shown in the last part of Tables 1, 2, 3, and 4 below.

Total and diffuse visible transmittance measurements are performed below 5 mm using a Varian spectrophotometer (Cary 500 Scan model) equipped with an integrating sphere. From these measurements, light transmittance (TL%) and scattering level (haze%) are determined according to the ASTM D 1003-13 standard (under light source C with observation at 2°). For the glass-ceramics of Examples 3 and 6, measurements are also performed on 4 mm thick samples. The obtained results are shown in parentheses in Tables 1 and 2, respectively.

Yellowing index (YI) is calculated according to transmittance measurement (color points) according to ASTM D1925 standard under light source C.

The average of the β-quartz crystals as well as the percentage of the β-quartz phase (relative to the total crystallized portion) are obtained using Rietveld structural analysis of the X-ray diffraction spectra. The numbers in parentheses indicate the average size of the crystals in nanometers.

CTEs (coefficient of thermal expansion) (between room temperature (25°C) and 300°C = CTE _{25°C -300°C} , and between room temperature (25°C) and 700°C = CTE _{25°C -700°C} ) for glass-ceramic samples in the form of rods. is measured with a high-temperature dilatometer (DIL 402C, Netzsch) at a heating rate of 3° C./min.

Examples 1 to 9 (Tables 1-A and 1-B) illustrate the present application. Example 3 is preferred. Examples 3, 7, 8 and 9 relate to glasses and glass-ceramics of similar compositions containing variable iron content (140 ppm, 100 ppm, 170 ppm and 220 ppm of Fe ₂ O ₃ , respectively). The iron acts essentially on the optical properties of the glass-ceramic of interest (neither ceramicization nor coefficient of thermal expansion). The optical properties of the glass-ceramic of Example 9 remain of interest.

Examples A to E (Tables 2-A and 2-B) are comparative examples.

Comparative example A corresponds to a glass-ceramic according to application WO 2013/171288. This composition of glass-ceramic does not contain any CaO, which is the condition required: 0.2 ≤ (MgO + K ₂ O + Na ₂ O)/Li ₂ O ≤ for the composition of glass-ceramic of the present application. 1 is not met. This glass-ceramic has a very negative CTE value of _{25°C -300°C} .

The composition of the glass-ceramic of Comparative Example B contains only 3.55% Li ₂ O. For a viscosity of 30 Pa.s, the temperature of glass (the precursor of the glass-ceramics) is high.

The composition of the glass-ceramic of comparative example C contains less than 63% SiO ₂ and 4.81% ZnO: for a viscosity of 30 Pa.s, the temperature of the glass (the precursor of the glass-ceramic) is only 1,573° C. , the glass-ceramic has a yellow color. The composition containing 4.81% ZnO, no CaO, no MgO, no K ₂ O, no Na ₂ O: CTE _{25°C -700°C} and CTE _{25°C -300°C} values of the glass-ceramic is not satisfactory.

The composition of the glass-ceramic of comparative example D differs from the composition of the glass-ceramic of the present application insofar as it does not fulfill the required conditions: 0.2 ≤ (MgO + K ₂ O + Na ₂ O)/Li ₂ O ≤ 1 . In fact, according to this composition: (MgO + K ₂ O + Na ₂ O)/Li ₂ O = 0.142. The CTE _{25°C -300°C} value of glass-ceramic is too low.

The glass-ceramic composition of Comparative Example E contains too much MgO. This glass-ceramic does not have CTE values and optical properties of interest.

Example One 2 3 4 SiO ₂ 65.029 64.856 65.026 65.334 Al ₂ O ₃ 22.76 22.69 22.72 22.83 Li ₂ O 4.09 4.05 4.18 4.2 MgO 0.31 0.18 0.31 0.31 ZnO 0.5 0.81 0.19 0.19 BaO 0.54 0.54 1.22 0 SrO 0 0 0 0 TiO ₂ 2.78 2.77 2.77 2.78 _ZrO2 1.84 1.83 1.83 1.84 SnO ₂ 0.3 0.3 0.3 0.3 Na ₂ O 0.26 0 0.26 0.26 _K2O 1.06 1.44 0.75 1.51 CaO 0.52 0.52 0.43 0.43 P ₂ O ₅ 0 0 0 0 _{Fe2O3 _} 0.011 0.014 0.014 0.016 (MgO+K ₂ O+Na ₂ O)/Li ₂ O 0.399 0.4 0.316 0.495 properties of glass T _{30Pa.s (} ℃) 1624 1624 T _liq (°C) 1330-1350 1340-1350 1328-1347 Viscosity in T _liq (Pa.s) 816-704 956-721 Properties of glass-ceramics TL (%) 84.8 84.9 85.7 (86.1) 85.3 Y.I. 9.2 9.6 8.1 (6.9) 9.9 Haze (%) 0.02 0.23 0.37 CTE _{25℃ - 300℃} (K ^-1 ) -2.3E-07 -2.6E-07 -3.4E-07 -1.3E-07 CTE _{25℃ - 700℃} (K ^-1 ) 1.3E-07 1.2E-07 1.1E-08 2.2E-07 β-quartz% (nm) 94 (27) 95 (27) 94 (28) 96 (27)

Example 5 6 7 8 9 SiO ₂ 65.155 66.285 65.03 65.023 65.018 Al ₂ O ₃ 22.77 22,19 22.72 22.72 22.72 Li ₂ O 4.14 4.18 4.18 4.18 4.18 MgO 0.31 0.94 0.31 0.31 0.31 ZnO 0.31 0 0.19 0.19 0.19 BaO 0.54 0 1.22 1.22 1.22 SrO 0 0,81 0 0 0 TiO ₂ 2.78 2.73 2.77 2.77 2.77 _ZrO2 1.84 1.92 1.83 1.83 1.83 SnO ₂ 0.3 0.30 0.30 0.30 0.30 Na ₂ O 0.26 0.19 0.26 0.26 0.26 _K2O 1.06 0 0.75 0.75 0.75 CaO 0.52 0.44 0.43 0.43 0.43 P ₂ O ₅ 0 0 0 0 0 _{Fe2O3 _} 0.015 0.015 0.010 0.017 0.022 (MgO+K ₂ O+Na ₂ O)/Li ₂ O 0.394 0.27 0.32 0.32 0.32 properties of glass T _{30Pa.s (} ℃) 1626 T _liq (°C) 1341-1356 Viscosity in T _liq (Pa.s) 774-624 Properties of glass-ceramics TL (%) 85.3 84.56 (85.56) 86.0 84.9 84.3 Y.I. 9.6 10.81 (8.63) 7.1 9.4 10.4 Haze (%) 0.32 0.33 0.35 0.19 0.25 CTE _{25℃ -300℃} (K ^-1 ) -2.7E-07 -1.8E-07 CTE _{25℃ - 700℃} (K ^-1 ) 1.0E-07 0.8E-07 β-quartz% (nm) 96 (28) 96 (29) 96 (28) 97 (27)

Claims (17)

With a glass-ceramic of the lithium aluminosilicate type containing a β-quartz solid solution as the main crystalline phase:
- The composition of the glass-ceramic, free of arsenic oxides, antimony oxides and rare earth oxides, except for unavoidable traces, contains the following components, expressed in percent by weight of oxides:
64 to 70% SiO ₂ ,
18 to 24% Al ₂ O ₃ ,
4 to 5% Li ₂ O,
0 to 0.6% SnO ₂ ,
>1.9 to 4% TiO ₂ ,
1 to 2.5% ZrO ₂ ,
0 to 1.5% MgO,
0 to 3% ZnO,
>0.3 to 1% CaO,
0.5 to 1.5% BaO,
BaO + SrO ≤ 3%,
0 to 1.5% Na ₂ O,
0 to 2% K ₂ O, where 0.2 ≤ (MgO + Na ₂ O + K ₂ O)/Li ₂ O ≤1,
0 to 3% P ₂ O _5,
SrO less than 1000 ppm,
less than 250 ppm Fe ₂ O ₃ ; and
- A glass-ceramic of the lithium aluminosilicate type, wherein the crystals present in the β-quartz solid solution have an average size of less than 40 nm. In claim 1,
The composition is a glass-ceramic of the lithium aluminosilicate type containing 0.1 to 0.6% of SnO ₂ . In claim 1 or 2,
The composition is a glass-ceramic of the lithium aluminosilicate type, containing more than 4% Li ₂ O and 2 to 3% TiO ₂ . In claim 1 or 2,
The composition is a glass-ceramic of the lithium aluminosilicate type, containing at least 0.1% of MgO and/or ZnO and less than 1% of P ₂ O ₅ . In claim 1 or 2,
The composition is a glass-ceramic of the lithium aluminosilicate type, free from arsenic oxide, antimony oxide, rare earth oxides, strontium oxide and phosphorus oxide, except for unavoidable traces, and containing the following components, expressed in percent by weight of oxide: :
64 to 68% SiO ₂ ,
20 to 24% Al ₂ O ₃ ,
>4 to 5% Li ₂ O,
0.1 to 0.4% SnO ₂ ,
2 to 3% TiO ₂ ,
1.5 to 2% ZrO ₂ ,
0.1 to 0.5% MgO,
0.1 to 1.5% ZnO. In claim 1 or 2,
- a luminous transmittance of greater than 81% for a thickness of 5 mm, a yellowing index of less than 14 for a thickness of 5 mm, and a haze percentage of less than 2.5% for a thickness of 5 mm; and
- Having a coefficient of thermal expansion of -3.5·10 ^-7 K ^-1 to +3.5·10 ^-7 K ^-1 between 25℃ and any temperature between 300 and 700℃ (CTE _{25℃ -[300∼700℃])} , a glass-ceramic of the lithium aluminosilicate type. An article consisting at least in part of a glass-ceramic according to claim 1 or 2 in a chimney window, fireplace fitting, stove or oven window, cooktop, shield, or non-combustible glazing. Lithium aluminosilicate glass, which is a precursor of the glass-ceramic according to claim 1 or 2, the composition of which makes it possible to obtain the glass-ceramic according to claim 1 or 2. In claim 8,
Lithium aluminosilicate glass, wherein the composition contains at least 0.1% ZnO and less than 1% P ₂ O ₅ . In claim 9,
A lithium aluminosilicate glass having a viscosity of 30 Pa.s below 1,640°C (T _@30Pa.s <1,640°C), a liquidus temperature below 1400°C and a liquidus viscosity greater than 300 Pa.s. delete delete delete delete delete delete delete

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